Reland "[base] Use NO_UNIQUE_ADDRESS in flat_tree"

This reverts commit 5d1496e0.

Reason for revert: https://bugs.llvm.org/show_bug.cgi?id=48048 got fixed.

Original change's description:
> Revert "[base] Use NO_UNIQUE_ADDRESS in flat_tree"
>
> This reverts commit f55e9981.
>
> Reason for revert: https://crbug.com/1144431
>
> Bug: 1144431
>
> Original change's description:
> > [base] Use NO_UNIQUE_ADDRESS in flat_tree
> >
> > This change makes use of NO_UNIQUE_ADDRESS in flat_tree, allowing the
> > removal of an internal class which just existed to make use of the empty
> > base optimization.
> >
> > Bug: 682254
> > Change-Id: Iabf91d4acee95c7ba4e929d23528d95b0ced1612
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2509757
> > Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> > Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#822948}
>
> TBR=dcheng@chromium.org,jdoerrie@chromium.org
>
> # Not skipping CQ checks because original CL landed > 1 day ago.
>
> Bug: 682254
> Change-Id: If3b15e15241e8f8b5d99b80caadbb25153648f37
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2514147
> Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
> Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#823192}

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1144431
Bug: 682254
Change-Id: I70549a9fa09c05c1528a3df6f317fe2ac042b814
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2553429Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830482}

base/containers/fixed_flat_map.h

@@ -99,7 +99,7 @@ constexpr fixed_flat_map<Key, Mapped, N, Compare> MakeFixedFlatMap(
     std::pair<Key, Mapped>(&&data)[N],
     const Compare& comp = Compare()) {
   using FixedFlatMap = fixed_flat_map<Key, Mapped, N, Compare>;
-  typename FixedFlatMap::value_compare value_comp(comp);
+  typename FixedFlatMap::value_compare value_comp{comp};
   internal::InsertionSort(data, data + N, value_comp);
   CHECK(internal::is_sorted_and_unique(data, value_comp));
   // Specify the value_type explicitly to ensure that the returned array has